SWIFT J1626.6-5156 is not a high mass X-ray binary
ATel #713; N. Rea (SRON Utrecht), V. Testa, G. L. Israel, A. Antonelli (INAF-OAR), P. Jonker (SRON Utrecht, CfA), T. Belloni, S. Campana (INAF-OAB), L. Stella (INAF-OAR)
on 27 Jan 2006; 10:56 UT
Credential Certification: Nanda Rea (N.Rea@sron.nl)
Subjects: Infra-Red, Optical, Request for Observations, Transient, Pulsar
on behalf of E. Molinari, G. Chincarini, F.M. Zerbi, S. Covino, G. Tosti, P. Conconi, G. Cutispoto, L. Nicastro, E. Palazzi, F. Vitali, F. D'Alessio, E. Meurs, P. Goldoni and the REMIR/ROSS collaboration
We report on infrared observations of the newly discovered transient X-ray pulsar SWIFT J1626.6-5156 (Palmer et al., ATEL #678, Markwardt & Swank, ATEL #679 and Campana et al., ATEL #688). The observations were performed in J, H and K filters every night from 2006 January 15th to 25th, with the REMIR Camera (field of view of 10'x10' and a pixel size of 1.22") mounted on REM, a 60-cm robotic telescope located at the La Silla Observatory (Chile).
We detect only one source in a 4" error circle around the Swift XRT position (Campana et al., ATEL #688), coincident with a 2MASS catalogued source, having the following magnitudes:
J = 13.47 +/- 0.03
H = 12.95 +/- 0.05
K = 12.59 +/- 0.07
The source magnitudes calibrated on the 2MASS itself are fully consistent with the 2MASS values, all the measurements giving differences below 0.1 mag, well within 1-sigma uncertainties. The differences between our measurements and the 2MASS magnitudes for the nights of January 20, 21 and 22 are reported in http://www.sron.nl/~nanda/swift1626.html . Our 3-sigma detection upper limits are J < 16.66, H < 15.75 and K < 15.29 .
Although this is the only infrared source in the X-ray error circle we cannot unambiguously identify it as the infrared counterpart of SWIFT J1626.6-5156, because no significant infrared variability
was detected which might have followed the X-ray outburst, and the XRT error circle is not small enough to statistically claim it as the right counterpart.
In any case, we can reliably say that the infrared magnitudes are too faint for the star being a Supergiant or a Be type star. Furthermore, after de-reddening (using the Nh~0.9x10^21 measured by the XRT and Predehl & Schmitt 1995) the infrared and optical magnitudes (optical magnitudes derived
from the DENIS database, I=14.58, R=15.9 and B=17.7), the source colours appear extremely red and highly inconsistent with an O or B type star (Cox 2002). On the other hand, with an Nh~0.9x10^21 cm^-2, any other star of these spectral types, even if at the edge of the Galaxy, should have been detected in the infrared band within our upper limits. This reliably rules out the possibility of SWIFT J1626.6-5156 being a canonical high mass X-ray binary pulsar. This makes this object a
probable low mass X-ray binary hosting a slowly rotating pulsar.
A more accurate determination of the SWIFT J1626.6-5156 position is needed in order to continue the investigation in the infrared and optical bands with bigger telescopes.
REM Results